Phosphorous removal from wastewater

ABSTRACT

A method for removing both carbon food and phosphorous pollutant by biochemical oxidation and chemical precipitation using oxygen gas in the presence of activated sludge, where most of the carbon food and pollutant are removed in a first covered zone with the addition of phosphorous-precipitating compound and under high food-to-biomass ratio, and the effluent water is further purified in a second covered zone under low food-to-biomass ratio.

United States atent [191 Stankewich, Jr.

[ Oct.9,1973

[ PHOSPHOROUS REMOVAL FROM WASTEWATER [75] Inventor: Michael J.Stankewich, Jr., North Tonawanda, N.Y.

[73] Assignee: Union Carbide Corporation, New

York, N.Y.

221 Filed: Nov. 13, 1972 211 App]. No.: 305,726

Related US. Application Data [63] Continuation-impart of Ser. No.249,159, May 1,

[51] Int. Cl C02c 1/10 [58] Field of Search 210/3-9, 210/11, 15, 18

[56] References Cited UNITED STATES PATENTS 3,480,144 11/1969 Barth eta]. 210/18 3,547,812 12/1970 McWhirter 210/7 3,547,815 12/1970 McWhirter210/7 OTHER PUBLICATIONS Eberhardt, W. A., et al., ChemicalPrecipitation of Phosphorous, etc., Jour. WPCF, Vol. 40, No. 7, pp.1239-1267 (1968) P.O.S.L.

Eckelberger, W. F., Jr., et al., Waste Water Treatment for CompleteNutrient Removal, Water and/Sewage Works Journal, Oct. 1969, pp. 396-402(GP. 176). Mulbarger, M. C., Report to FWQA, Modifications of ActivatedSludge, etc. Aug. 1970. (Paper presented to 432 Conf. of WPCF, BostonMass., Oct. 4-9, 1970).

Long/et al., Soluble Phosphorous Removal in Activated Sludge Process,Part. I, EPA Project No. 17010 EIP, Aug. 1971.

Primary Examiner-Michael Rogers AttorneyJames C. Arvantes et al.

[5 7] ABSTRACT A method for removing both carbon food and phosphorouspollutant by biochemical oxidation and chemical precipitation usingoxygen gas in the presence of activated sludge, where most of the carbonfood and pollutant are removed in a first covered zone with the additionof phosphorous-pmcipitating compound and under high food-to-biomassratio, and the effluent water is further purified in a second coveredzone under low food-to-biomass ratio.

15 Claims, 2 Drawing Figures PATENIEDHET w 3.764.524

SHEET 1 UF 2 2lo 20b 21c PHOSPHOROUS REMOVAL FROM WASTEWATERCROSS-REFERENCE TO RELATED APPLICATION This application is acontinuation-in-part of Ser. No. 249,l59 filed May I, 1972 in the nameof Michael J. Stankewich, Jr. entitled Nitrification of BOD- ContainingWater.

BACKGROUND OF THE INVENTION This invention relates to a method fortreating wastewater by oxygenation and chemical precipitation to removeboth carbon food and phosphorous pollutants from such water and therebyminimize the latters oxygen demand and avoid eutrophication of thereceiving waters.

Phosphorous and nitrogen compounds which are present in water inappreciable concentrations, are important nutrients. The discharge oflarge quantities of these nutrients into natural waters promotes thegrowth of algae and results in eutrophication of lakes and similardeterioration of water quality in receiving streams. Although bothnitrogen and phosphorous compounds are essential to the growth of algae,phosphorous is generally considered to be a more critical nutrientbecause, unlike nitrogen, it can only be supplied by influx ofphosphorous-containing compounds entering the receiving body of water.In contrast, certain species of algae, particularly the nuisanceblue-greens, are capable of satisfying their nitrogen demand by directutilization of the atmospheric nitrogen. Thus, control of eutrophicationmay best be achieved through control of phosphorous.

Phosphorous is usually present in the wastewater in the form of organicphosphorous, inorganic condensed phosphates, and orthophosphates. Mostof the organically-bound phosphorous compounds in the wastewater arepresent as particulate organic matter and as bacterial cells. Verylittle is known about the dissolved organic phosphorous compounds whichare the byproducts of bacterial metabolishm and cell lysis. Inorganiccondensed phosphates such as tripolyphosphate and pyrophosphateoriginatemainly in household detergents. Orthophosphate is an end product ofmicrobial degradation of phosphorous-containing organic compounds;orthophosphate is also excreted in urine, and is the product ofenzymatic hydrolysis of condensed phosphates. Phosphorous in theorthophosphate form is more readily available for biologicalutilization. The concentrations of the various forms of phosphorous indomestic wastewater are subject to wide hourly and daily fluctuations.Wastewaters received at or discharged from different plants also containvarying concentrations of phosphates depending on the type of communityserved and the nature of the biological treatment process employed.

The most widely practiced method of wastewater treatment is biochemicaloxidation and in particular the secondary activated sludge system. Inrecent years this system has been vastly improved by the use of highpurity oxygen gas as the oxidant in a series of closed tanks, preferablywith staging of gas and liquor from tank to tank in the manner describedby U. S. Pat. No. 3,547,515 to J. R. McWhirter.

Recognizing that the objective is to remove all pollutants fromwastewater, including the carbonaceous, nitrogenous and phosphorousforms, it is unfortunate that the process steps and conditions bestsuited to the removal of one pollutant are detrimental to the effectiveremoval of another, so that optimum overall performance may not beachieved in a single treatment step. For example, it has been shown inmy abovereferenced U. S. patent application Ser. No. 249,159 that thegrowth rate of carbonaceous consuming microorganisms is far more rapidthan nitrogenous consum ing forms. As a result, the high sludge wastingrate demanded by the carbonaceous consuming biomass prevents buildup ofan efi'ective nitrifying biomass within the same sludge. If phosphorousremoval is also practiced, solids production in the system is furtheraugmented and this not only aggravates the depletion of nitrifyingmicroorganisms but also creates a similar maintenance problem forcarbonaceous consuming microorganisms.

The requirement that phosphorous be removed in the activated sludgeprocess can impose exceptional problems both to the biological processand to its purification effectiveness. Phosphorous is an essentialnutrient for the growth of the microorganisms on which the activatedsludge process depends, and a fraction of the phosphorous in thewastewater will be removed by the production anddisposition of excessbiomass. However, the fraction thus removed is minor and other steps areusually needed in order to meet purity standards. The most practicalmethod for reducing phosphorous content is by chemical precipitationwith a metal compound such as aluminum sulfate or ferric chloride.

In phosphorous precipitation, there are other competing chemicalreactions within the treatment process which consume a portion of thechemical additive, and dosages substantially in excess of thestoichiometric ratio with phosphorous must be applied in order to obtaindesired removals. Regardless of the chemical reaction involved,essentially all ofthe chemical additive will convert to an insolubleproduct and therefore a considerable quantity of chemical solids will beproduced.

Three basic procedures have been proposed for precipitating phosphorousin association with activated sludge treatment. These procedures are (a)pretreatment by precipitation and removal upstream of activated sludgetreatment, (b) post-treatment by precipitation and removal downstream ofactivated sludge treatment, and (0) combined treatmentby precipitationand removal in situ of activated sludge treatment.

Combined treatment is practiced by adding phosphorous-precipitatingcompounds directly to the mixed liquor in the activated sludge treatmentstep and thereby removing both carbonaceous matter and phosphoroussimultaneously and in the same equipment. Unfortunatly, the combinedproduction of chemical solids (primarily phosphorous salts) andbiological sludge is so great that the system is overburdened. Inpretreatment or post-treatment, the chemical solid is withdrawn from thepoint of precipitation and is passed directly to waste. However, incombined treatment, the biological sludge must be retained andrecirculated within the system. With combined phosphorous removal, theheavy inert chemical sludge will also accumulate in admixture with thebiological solid and may comprise 50 percent of the total solids. Theincrease in solids wasting in the combined treatment system results in areduction in the carbonaceous consuming microorganisms in the systemwith consequent reduction in BOD -removal capability. The high solidswasting rate necessitated by the more rapid accumulation of the combinedsolids will also seriously reduce or destroy any nitrificationcapability of the activated sludge process. The latter effect occursbecause nitrifying bacteria exhibit slow growth rates, and anyappreciable wasting of sludge will virtually deplete the system of allsuch slow-growing bacterial species. Finally, the unavoidableaccumulation of chemical solids to high levels within the system willimpair (rather than aid) flocculation and clarification, and theeffluent will become turbid and milky. The effluent suspended solidswill increase to perhaps 30-50 ppm., well above levels observed ineffluent from modern activated sludge systems without combinedphosphorous removal. Control of dosage rates and pH, while effective inpreventing turbidity in post-treatment phosphorous removal, will not perse avoid turbidity in combination treatment.

An object of this invention is to provide an improved method forremoving both carbon food and phosphorous from wastewater in a combinedactivated sludgetype system.

Another object of this invention is to provide an activated sludge-typesystem capable of combined treatment of wastewater to removecarbonaceous, nitrogeneous and phosphorous pollutants. Other objects andadvantages of this invention will be apparent from the ensuingdisclosure and appended claims.

SUMMARY This invention relates to a method for treating wastewater byaeration in contact with activated sludge, settling sludge from theaeration and recycling sludge to the aeration zone as the activatedsludge wherein the carbon food in the wastewater is biochemicallyoxidized with at least 50 percent oxygen(by volume) feed gas. In theimprovement of this invention wherein phosphorous pollutant is removed,a wastewater feed stream containing carbon food and soluble phosphorouspollutant, at least 50 percent oxygen (by volume) feed gas and firstsolids recycle are introduced to a first aeration zone having a closedoverhead gas space, for

mixing and simultaneous recirculation of one fluid against the otherfluids. A phosphorous-precipitating compound selected from the groupconsisting of ferric chloride and aluminum sulfate is also introducedand the aforementioned introducing, mixing and recirculating are atrates such that: (a) insoluble chemical solids are produced includingprecipitating phosphorous salt, and the phosphorous-precipitatingcation/phosphorous pollutant molar ratio is l.2l.8, (b) the food/biomassratio is maintained at 0.8-2.5 pounds BOD lday X pound volatilesuspended solids (MLVSS), (c) the volatile suspended solidsconcentration (MLVSS) is at least 2,000 ppm., (d) the total mixing andfluid recirculation energy expended in a liquor terminal flow section ofsaid first aeration zone having liquor contact time of at least 10minutes does not exceed 0.3 horsepower/1,000 U.S. gallons of terminalflow section liquid capacity including a high shear part of said totalmixing and recirculation energy not exceeding 0.25 horsepower/1,000 U.S.gallons, (e) the total mixing and fluid recirculation energy expended inthe liquor flow section of said phosphorous-precipitating compoundintroduction does not exceed 0.3 horsepower/1,000 U.S. gallons of saidliquor flow section liquid capacity including a high shear part of saidtotal mixing and recirculation energy not exceeding 0.25 horsepower/1,000 U.S. gallons, (f) the dissolved oxygen concentration in saidliquor terminal flow section is at least 2 ppm., (g) the pH of saidliquor in said first aeration zone is 5.5-7.0, and (h) the total liquorcontact time in said first aeration zone does not exceed minutes.

Oxygen-depleted aeration gas of at least 20 oxygen (by volume) contentis released from the first aeration zone overhead gas space. Partiallyoxygenated liquid is also discharged from the first aeration zone andseparated into partially treated effluent water still containing atleast 25 ppm. BOD, and unconsumed phosphorous-precipitating cation, and.settled solids having a chemical solids/total solids weight ratio of atleast 0.25. Part of the settled solids are returned to the firstaeration zone as the first solids recycle.

The partially treated effluent water, at least 50 percent oxygen (byvolume) feed gas, and second solids recycle are introduced to a secondaeration zone having a closed overhead gas space. The fluids are mixedin the second aeration zone and the phosphorousprecipitating cationconcentration is maintained therein so that the chemical solids/totalsolids weight ratio is at least 0.05. Also one fluid is simultaneouslyrecirculated in the two aeration zones against the other fluids, and theaforementioned introductions, mixing and recirculating are at rates suchthat: (i) additional insoluble chemical solids are formed from thephosphorous-precipitating cation, (j) the food/biomass ratio ismaintained at 015-08 pounds BOD /day X pound volatile suspended solids(MLVSS) and the ratio of first to second aeration zone food/biomassratio is at least 2, (k) the total mixing and fluid recirculation energyin the liquor introductory flow section of said second aeration zonehaving liquor contact time of at least 10 minutes does not exceed 0.30horsepower/1,000 U.S. gallons of second areation zone liquid capacityincluding a high shear part of said total mixing and fluid recirculationenergy not exceeding 0.25 horsepower/1,000 U.S. gallons, (1) the totalmixing and fluid recirculation energy expended in a liquor terminal flowsection of said second aeration zone having liquor contact time of atleast 10 minutes does not exceed 0.25 horsepower/1,000 U.S. gallons ofterminal flow section liquid capacity including a high shear part ofsaid total mixing and fluid recirculation energy not exceeding 0.20horsepower/1,000 U.S. gallons, (m) the pH of said liquor in said secondaeration zone is 5.5-7.0, (n) the dissolved oxygen concentration in saidliquor terminal flow section is at least 2 ppm. and (o) the total liquorcontact time in said second aeration zone does not exceed 240 minutes.

Oxygen-depleted gas of at least 20 percent (by volume) content isreleased from the second aeration zone overhead gas space. Furtheroxygenated liquor is also discharged from the second aeration zone andseparated into product effluent water and settled solids having chemicalsolids/total solids weight ratio of less than 0.25. Part of the settledsolids are returned to the second aeration zone as the second solidsrecycle.

As used herein, the term BODQ refers to the biochemical oxygen demandfor a given sample measured after a five day incubation period inaccordance with the standardized procedure outlined in Standard Methodsfor the Examination of Water and Wastewater, American Public HealthAssociation, Inc., New

York 1971 (Pages 489-495). Except as specifically indicated, all othermeasurements set forth hereinafter were made following the standardizedprocedures outlined in this publication. The BOD measurement includescarbon food (appearing as soluble material), non-viable material andcarbon-consuming microorganisms (both appearing as volatile suspendedsolids), but not nitrogen food or nitrogen-consuming microorganisms.

As used herein, the expression food/biomass ratio? is the ratio of thesum total of the carbon food, nonviable material and carbon-consumingmicroorganisms to volatile suspended solids, i.e., pounds BOD /day poundvolatile suspended solids (MLVSS) in an aeration zone. Also as usedherein the term chemical solids" refers to the inorganic solids formedas a result of adding the phosphorous-precipitating compound to thefirst and second aeration zone. These inorganic solids include theferric or aluminum cation from the added compound and the anion may forexample include the phosphorous pollutant, or the chemical solid may bean oxide or hydroxide of the cation. The term total solids refers to themixture of both chemical solids (as defined above) and the suspendedsolids normally present in the activated sludge process which are of abiological, carbonaceous origin, plus inert solids in the wastewaterfeed.

This invention realizes the aforestated objects and as compared to priorart activated sludge systems, tests have demonstrated that this methodis capable of removing substantially all the aforesaid wastewatercontaminants in tankage whose total volume is no greater than the volumerequired for equivalent treatment in a single step high purity oxygensystem, but without phosphorous removal.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view taken incross-sectional elevation of apparatus capable of practicing oneembodiment of the invention wherein the first and second aeration zonesemploy single gas-liquor contact zones approaching plug flow conditions.

FIG. 2 is a schematic view taken in cross-sectional elevation ofapparatus capable of practicing another embodiment wherein the first andsecond aeration zones each comprise a multiplicity of separate subzonesproviding staged gas-liquor contacting.

Although the liquor is preferably staged through a series of sub-zonesin the first aeration zone, the latter may be a non-partitioned chamberwherein the liquor is completely mixed, as for example described in U.S. Pat. No. 3,547,812, to J. R. McWhirter, incorporated herein to theextent pertinent. In this embodiment, the aforementioned total energyexpended/1,000 U.S. gallons liquid capacity ratio for the liquorterminal flow section of the first aeration zone applies to the entirezone.

DESCRIPTION OF PREFERRED EMBODIMENTS United States Pat. No. 3,547,812issued Dec. l5,

1970 to J. R. McWhirter describes an improved system for biochemicallytreating BOD-containing water by at least 60 percent (by volume) oxygengas in contact with active biomass (activated sludge) to form liquor.The mixing is continued while simultaneously maintaining: (a) the oxygenfeed gas to mixing plus gasliquor contact energy ratio at 0.03-0.40 lb.moles oxygen per horsepower hour of energy supplied, (b) the aerationgas above the liquor at oxygen partial pressure of at least 300 mm. Hgbut below 80 percent oxygen (by volume) while consuming at least 50percent (by volume) of the feed gas oxygen in the liquor, (c) thedissolved oxygen concentration of the liquor at below percent ofsaturation with respect to the oxygen in the aeration gas but aboveabout 2 ppm. and (d) continuously recirculating one of the aeration gasand liquor fluids in intimate contact with the other of the fluids inthe aeration zone. Oxygenated liquor is thereafter withdrawn from theaeration zone and preferably separated into cleaned effluent water andactivated sludge, a portion of the latter being recycled to the aerationzone.

U.S. Pat. No. 3,547,815 issued Dec. 15, 1970 to .I. R. McWhirterdescribes another improved system for biochemically treatingBOD-containing water by at least 50 percent (by volume) oxygen gas incontact with active biomass. In this McWhirter gas-staged system, theoxygen feed and other fluids are mixed and one fluid is simultaneouslycontinuously recirculated in a first gaseous oxygen stage to form firstoxygenated liquor and first unconsumed oxygen-containing gas. The latteris discharged from the first stage and mixed with aqueous liquid-solidin a second stage and one of the fluids is also continuouslyrecirculated against the other fluids in the second stage. Although onlytwo gas stages are essential, it is often desirable to provideadditional gas stages and operate same in a manner analogous to thefirst two stages. If the system is within an enclosed chamber, it isalso preferred to flow the oxygenated liquor from stage-to-stagecocurrent (in the same direction as) the gas staging.

Both of these oxygen biochemical treatment systems offer importantadvantages as compared with conventional air aeration of wastewater, forremoval of carbon food. The advantages for example include smalleraeration equipment, lower power costs, lower capital investment, lowersludge handling costs and less land space. However, these systems havethe same previously enumerated disadvantages as air aeration systemswhen used for combined removal of carbon food and phosphorouspollutants. The high sludge wasting rate reduces the BOD /removalcapability and also the nitrification capability if the objective is toremove all such pollutants from the wastewater.

It has now been discovered that these problems can be overcome by a twostep method in which the wastewater is first treated in a first aerationzone using oxygen gas as taught by the referenced McWhirter patents, toremove at least a major portion (typically percent for municipal sewagein the high food/biomass ratio embodiment) of the wastewater oxygendemand due to carbon food (as measured by customary BOD determination).A phosphorous-precipitating compound selected from the group consistingof ferric chloride and aluminum sulfate is also introduced for mixingwith the first aeration zone liquor. These compounds are soluble inwastewater, produce trivalent cations in solution, and the cationicportion has an affinity for phosphorous pollutants so as to forminsoluble phosphorous salts therewith. The added water soluble compoundsalso form. acid solutions by hydrolysis. The aluminum sulfate, Al (SO,,)n H O, will hereinafter be referred to as alum" and the commercial gradeof alum containing minor quantities of impurities has been found quitesatisfactory in the practice of this in vention. Alum usually includes14 or 18 molecules of water bound in the crystalline form. Sodiumaluminate, NaAl(Ol-I),, is not a suitable phosphorousprecipitatingcompound because at the conditions prevailing in aeration zones withclosed overhead gas space and high purity oxygen gas, sodium aluminateforms basic solutions and impairs flocculation of the chemical solids.

To obtain complete precipitation of the phosphorous pollutant, it isnecessary to introduce a relatively large stoichiometric excess ofphosphorous-precipitating cation, i.e., the Al or Fe cation/phosphorouspollutant molar ratio is 1.2-1.8. For purposes of this relationship,phosphorous pollutant comprises all phosphorous contained in thewastewater which is analytically detectable by the method specified inthe previously referenced Standard Methods for the Examination of waterand wastewater. In particular, the non orthophosphate forms ofphosphorous pollutant are first converted to the ortho form by thepersulfate digestion method described in the 13th edition, page 526, andthe ortho phosphate determination is made using theaminonaphtha-sulfonic acid method described in the 12th edition, page231.

The phosphorous-precipitating compound is introduced in a liquor flowsection (either in the first aeration zone or between this zone and itsclarifier) where the total mixing and fluid recirculation energy isrelatively low to avoid mechanical damage to thephosphorous-precipitating floc particles, i.e. where the total energyexpended does not exceed 0.30 horsepower/1,000 U.S. gallons of theliquor flow section liquid capacity including a high shear part of thetotal mixing and recirculation energy not exceeding 0.25horsepower/1,000 U.S. gallons. Excessive attrition will cause dispersionof floc particles and result in poor clarification even in normalpractice of the activated sludge system without phosphate removal. Whenphosphorous-precipitating compounds are employed within the mixed liquoraeration zone, the chemical-biological floc particles are much moresusceptible to mechanical damage and dispersion.

In the high purity oxygen aeration gas systems described in theaforementioned McWhirter patents, energy is needed to both mix theliquor to keep the solids in suspension and recirculate one fluidagainst the other fluids within the closed aeration zone to promote masstransfer between gas and liquid. The energy needed for a particularsystem depends on such factors as the BOD; content of the wastewater,the type of mixing-fluid recirculation equipment used, thebiodegradability of the wastewater, and the food/biomass ratio. Forexample, if a surface impeller is used to perform both the mixing andfluid recirculation functions, the power needed to achieve satisfactorysolids suspension and oxygen dissolution is relatively high. Moreover,the surface impeller is a high shear device and imposes greater damageto floc particles than other devices. Another suitable mixing-fluidrecirculation assembly is a sub-surface rotating sparger for introducingthe oxygen gas with a mixing propeller positioned above the impeller,preferably on the same shaft and also below the liquor surface. In thisassembly the gas is withdrawn from the overhead space by a pump andreturned to the sparger. Only the sparger arms produce high shear on thesolids and the major portion of the energy consumed by the rotatingassembly is to operate the propeller which is a low-shear deviceproducing very little floc damage. Still another efficient fluidrecirculationmixing system is the combination of a surface impeller anda submerged propeller, wherein the surface impeller is sized and poweredsolely to perform the liquoragainst-gas recirculation function while thesubmerged propeller performs the liquid-solids mixing. As in the case ofthe rotating sparger, the surface impeller is a high shear device whilethe bottom propeller is low shear. It should be noted that the chemicalsolids produced in phosphorous removal are relatively heavy anddifficult to hold in suspension. The power required for liquid-solidsmixing is significantly higher when phosphorous salts are precipitatedin the manner of this invention, than when phosphorous removal is notpracticed.

Biodegradability of the wastewater affects power requirements. If thewastewater is readily degradable, the oxygen demand nearer the feed endof the first zone will be relatively high and a relatively steep declinein power requirement might be expected between wastewater feed andeffluent ends of the first aeration zone. If the wastewater is notreadily biodegradable, reaction rates will be slower and the power (andoxygen) requirement will tend to be spread more uniformly fromend-to-end of the first aeration zone.

Since the major part of the chemical and biological solids are producedin the first aeration zone, the energy requirements in the secondaeration zone are lower and the biological reaction rates are lower.

As previously indicated, the first and second aeration zones maycomprise single aeration chambers, and the basic geometry may beadvantageously chosen to simulate plug flow with the liquid(liquid-solid) continuously moving from the wastewater inlet end to theeffluent end. This relationship simulates plug flow and suppressesback-mixing along the chamber length. In this event, a series ofliquid-solid mixing means are preferably spaced along the liquid-solidflow path. However, in the preferred practice of this invention, eachzone is divided into a multiplicity of separate subzones with all feedfluids introduced to a first sub-zone of the first aeration zone formixing and simultaneous fluid recirculation therein to form a firstpartially oxygenated liquor and a first oxygen-depleted aeration gas.These fluids are separately withdrawn and each introduced to a secondsub-zone for further mixing and simultaneous fluid recirculation to forma second partially oxygenated liquor and second further oxygendepletedaeration gas. The fluids are also separately withdrawn from the secondsub-zone and each introduced to any remaining sub-zones of the firstaeration zones for further mixing and fluid recirculation in the samecocurrent flow direction as the first and second sub-zones. In thisembodiment the phosphorous precipitating compound is preferablyintroduced to the final sub-zone of the first aeration zone. The secondaeration zone is preferably arranged and constructed to provide forcocurrent flow from the inlet to the effluent end in an analogousmanner. In this staged flow embodiment, mixing and fluid recirculationmeans are needed in each sub-zone.

As used herein, the expression liquor terminal flow section" of thefirst aeration zone refers to the end from which the partiallyoxygenated liquor and oxygendepleted aeration gas are discharged for thestaged liquor and semi plug-flow" embodiments. In the completely mixedliquor embodiment, there are no subzones. Conversely, the liquorintroductory flow section of the first aeration zone refers to theopposite end at which the wastewater, first solids recycle and at least50 percent oxygen feed gas are introduced. The liquor terminal flowsection of the second aeration zone refers to the effluent end of thatzone from which the further oxygenated liquor and oxygen-depletedaeration gas are discharged. The liquor introductory flow section of thesecond aeration zone refers to the opposite end at which the partiallytreated effluent water, at least 50 percent oxygen (by volume) feed gasand second solids recycle are introduced. When the aeration zone inquestion is divided into sub-zones, the liquor introductory flow sectionis the first sub-zone to which the wastewater or partially treatedeffluent water is introduced, and the liquor terminal flow section isthe last sub-zone from which the partially oxygenated liquor or furtheroxygenated liquor is discharged.

One requirement of this invention is that the total mixing and fluidrecirculation energy expended in the liquor terminal flow section havingliquor contact time of at least minutes does not exceed 0.3horsepower/1,000 U.S. gallons of first aeration zone liquid capacity.Moreover this energy includes a high shear part which does not exceed0.25 horsepower/l,000 U.S. gallons. This energy includes that necessaryto drive the motors which in turn power any surface impellers,sub-surface propellers, rotatable spargers, and gas recirculation pumps.It does not include the energy needed to separate air (to form theoxygen aeration gas), and drive the gas and liquor from the introductoryflow section to the terminal flow section of the first aeration zone.The expression liquor contact time" refers to the total period aparticular quantity of liquor (liquid-solid) is mixed with oxygen gas.It is based on the wastewater plus solids recycle passing through aparticular liquor flow section, and is calculated by dividing the flowsection volume by the liquor flow rate therethrough. For example, if theliquor flow rate through the terminal flow section is 10 milliongallons/minute, the volume of the terminal flow section is 0.14 millionU.S. gallons, and the energy input comprises 35 fii gh-shar horsepowerfor surface impeller and 7 low-shear horsepower for a submergedpropeller, the total mixing and fluid recirculation energy is 0.30horsepower/1,000 U.S. gallons liquid capacity and the liquor contacttime in the terminal flow section is 20 minutes. The aforementionednumerical limits represent the maximum energy levels which can betolerated by the combined chemical-biological (total) solids withoutsustaining excessive dispersion so as to prevent effective settling inthe clarifier. The energy level must usually be at least 0.08horsepower/1,000 U.S. gallons capacity in order to maintain the solid inuniform suspension.

The use of oxygen-rich aeration gas (as distinguished from air) permitsoperation within this power range. Moreover, the oxygen demand in theterminal flow section of the first aeration zone is substantially lessthan in the introductory flow section and less energy must be expendedin the terminal zone for fluid recirculation (needed for mass transfer).A low energy terminal flow section in the first aeration zone willprovide opportunity for reconstitution of floc which may have beendamaged in upstream sections of the zone where oxygen demand is higher.In a preferred embodiment, the total mixing and fluid recirculationenergy expended in the first aeration zone liquor terminal flow sectiondoes not exceed 0.25 horsepower/1,000 U.S. gallons including a highshear part of such energy not exceeding 0.20 horsepower/1,000 U.S.gallons.

As previously indicated, the phosphorousprecipitating compound isintroduced in a liquor flow section where the total mixing and fluidrecirculation energy is relatively low, i.e., does not exceed themaximum allowable in the liquor terminal flow section. It is preferablythough not necessarily introduced to the liquor terminal flow section.Since the precipitation reaction is very rapid, thephosphomus-precipitating compound may be added to the effluent channelwhich transports the partially oxygenated liquor from the terminal flowsection to the clarifier. Usually this channel is an open trough withoutmechanical mixing so that the energy level due to gravity flow is wellbelow the aforementioned maximum energy level.

The food/biomass ratio in the first aeration zone is maintained at arelatively high level of 0.8-2.5 pounds BOD /day pound MLVSS and thevolatile suspended solids concentration is maintained at least at 2,000ppm. These parameters are most readily controlled by varying the speedof the pump recycling first solids from the first clarifier to the feedend of the zone. This is because the wastewater feed rate in a treatmentplant is usually not controllable although it usually variesconsiderably during a 24 hour period. The food/biomass ratio is ofcourse related to both volume of the aeration zone and wastewaterstrength. For a given wastewater flow rate and BOD strength, and a givenconcentration of volatile suspended solids under aeration, thefood/biomass ratio is inversely related to the liquor contact time inthe aeration zone. This high food/biomass ratio in the first zone isneeded to insure that the partially oxygenated effluent water enteringthe second aeration zone has been only partially depleted of its carbonfood in the first aeration zone, i.e., still contains at least 25 ppm.BOD and unconsumed phosphorous-precipitating compound.

Whereas the use of high food/biomass ratios on the order of 0.8-2.5pounds BOD /day X lb. MLVSS are reported in air aeration systems toresult in poor settleability and low density return sludge, the use ofat least 50 percent oxygen in the first aeration zone of this systemunder such high F/M values results in good settleability and highdensity first solids recycle. Accordingly, high total mixed liquorsolids concentration (MLSS) can be achieved with low first solidsrecycle/wastewater volume ratios in the first zone, even at highF/Mvalues. This permits a significant reduction in liquor contact timeand reactor volume.

It should be noted that the relatively high food/biomass ratiomaintained in the first aeration zone requires a relatively low totalliquor contact time, i.e., not exceeding minutes based on combinedwastewater and first solids recycle.

Nitrification (the assimilation of nitrogen food by nitrogen-consumingbacteria) will usually not occur to any appreciable extent in the firstaeration zone. The high food/biomass ratio and the precipitation ofchemical solids result in a relatively high yield of excess total solidsfrom the first aeration. As a result the nitrifying bacteria are wastedat too high a rate to sustain a significant concentration of these formsin the biomass. The

dissolved oxygen concentration in the liquor terminal flow section ismaintained at least at 2 ppm. to insure a sufficient driving force forthe biochemical oxidation.

To obtain the high degree of solids removal from the partially treatedeffluent water in the second aeration zone (discussed hereinafter indetail) the liquor pH must be maintained in the range of 5.5-7.0 in boththe first and second zones, and preferably in the range of 5.5-6.5 Withair aeration, the normal pH of liquor undergoing treatment is relativelyhigh, e.g., 7.0 to 8.0, and acids such as sulfuric must be employed todepress the pH to within the foregoing optimum range. The cost ofchemicals to modify the pH is significant, and expensive controls arerequired in order to monitor and adjust flows of acid in accordance withfluctuating pH value and buffering capacity of the incoming wastewater.However, in the oxygen aerated activated sludge system employing amultiplicity of sub-zones each with a closed overhead gas space, the pHof the mixed liquor is inherently maintained within the range desiredfor phosphate precipitation and pH-adjusting chemicals are usually notrequired. The lower pH characteristic of oxygen aerated mixed liquor isdue to the high content of carbon dioxide maintained within therecirculated fluids. The CO content, and hence the pH, is controllableby regulating the rate at which CO -laden aerating gas is vented andreplenished with fresh oxygen. ln the air system, the CO is continuallystripped from the liquid by the once-through flow of a very large volumeof air.

A preferred characteristic for the first aeration zone is a liquorintroductory flow section having liquor contact time of at least 10minutes for high-rate cell syn thesis and BOD removal by virtue ofmixing the carbon food-containing wastewater and the carbonconsumingbacteria containing first solids recycle and oxygen absorption from theaeration gas. Also, the total liquor contact time (for wastewater plusfirst solids recycle) in the first aeration zone does not exceed 180minutes. One reason for the latter requirement is that the firstaeration zone must be operated in a manner such that the partiallyoxygenated effluent water is incompletely treated and in fact containsat least 25 ppm. BOD and unconsumed phosphorous-precipitating compound.

The partially oxygenated liquor is discharged. from the liquor terminalflow section of the first aeration zone and separated into theaforementioned partially treated effluent water and settled solidshaving a chemical solids/total solids weight ratio of at least 0.25 andpreferably less than 0.50. This ratio depends upon the BOD phosphorouspollutant and non-viable solids concentrations in the wastewater, alongwith the previously discussed process variables. For example, it hasbeen determined that with a wastewater feed stream containing 205 ppm.BOD ppm. soluble phosphorous pollutant, 72 ppm. non-biodegradablesolids, an aluminum cation/phosphorous molar ratio of 1.3,foodto-biomass ratio of 1.25, ppm, residual BOD in partially treatedeffluent water, a cell yield coefficient of 0.6 (lbs. biological solidsproduced/lb. BOD removed), and 0.57 lbs. chemical solids produced/lb.dry alum added, the chemical solids/total solids weight ratio of thesettled solids from the first aeration zone will be about 0.35 in astaged oxygen aeration system. However, if the phosphorous pollutantconcentration in the wastewater feed is only 6 ppm. and the othermethods outlined in the previously identified Standard Methods for theExamination of Water and wastewater for determining total solids (MLSS)and volatile solids (MLVSS). This is because heating the sample from C.(the MLSS test temperature) to 550C. (the MLVSS test temperature) notonly volatilizes the organic solids but also drives off at least asubstantial portion of the bound water of the chemical solids. Theresidue after baking at 550C. contains not only the feedwaternon-biodegradable solids but also the dehydrated chemical solids.

At least two modified procedures are suitable for determining thechemical solids content of the settled solids. The procedures are basedon a technique described in Humenick, MJ. and Kaufman, W.J. AnIntegrated Biological-Chemical Process for Municipal WastewaterTreatment, Proc. 5th International Water Pollution Research Conference,July-August 1970, Pergamon Press Ltd. (1971). According to thistechnique, the sample of mixed water and solids is acidified to pH of 2for 10 minutes before normal filtration and drying. Acidificationdissolves the chemical solids without significantly reducing the organicsolids, thus providing measures of the MLVSS comparable to thosenormally used in operating practice. This acid-MLVSS test can beemployed in a special test procedure to isolate the chemical solidsfraction.

According to the first of the modified procedures, an activated sludgetreatment step is operated and stabilized without phosphorous removaland operated and stabilized in a separate but otherwise identical testwith phosphorous removal. MLSS and MLVSS determinations are obtained forboth modes of operation the MLSS determinations being made at normal pHand the MLVSS determinations being made using the acid- VSS procedure.The resulting data may be used in formula (1) to determine the chemicalsolids/total solids weight ratio of the settled solids, as follows:

Let A MLVSS/MLSS ratio of normal solids without phosphorous removal BMLVSS/MLSS ratio of combined solids with phosphorous removal C chemicalsolids expressed as a fraction of the total solids in B Then C=1B/A Theforegoing modified procedure is difficult and slow because in a singlesystem, it requires operation and stabilization of an activated sludgeprocess over two periods of time. Even if the periods were consecutive,normal variations in wastewater content and operating conditions wouldintroduce unavoidable error. A preferred modified procedure fordetermining chemical solids content in the total solids which issimpler, faster and more reliable merely involves making two MLSSdeterminations at 105C one at normal pI-I (MLSS and the other at pH of 2(MLSS The difference between the two tests is accounted for by thechemical solids which are resolubilized under acid conditions. Thechemical solids fraction C is calculated as follows:

c (MLSSN MLSSQ/MLSSN At the present time, the latter method is the bestavailable for determining chemical solids fraction. Its accuracy dependsupon two assumptions: (1) acidification redissolves all the chemicalsolids, and (2) acidification does not dissolve any inert solidsincident in the wastewater. In all probability, neither assumption is100 percent accurate, but the deviations are probably minor and will notseriously affect the operable range of the invention as affected bydeterminations of the chemical solids fraction.

The quantity of total solids formed in the first aeration zone and hencethe part to be discarded as not needed for recycle, is relatively largeas compared with a system wherein a phosphorous-precipitating compoundis not added. There are at least two reasons for this phenomena. Thehigh food/biomass ratio maintained in the first aeration zone isconductive to high growth rate of bacteria, and in addition tends toreduce the fraction of total bacteria which are oxidized in this zone byendogenous respiration. Secondly, the phosphorous precipitant graduallyincreases in the total solids to constitute as much as 50 percent byweight of the total solids. By way of illustration, in the normalpractice of the staged oxygen aerated activated sludge system, theexcess solids produced for disposal from the system may comprise 0.3-0.6lbs. solids/lb. BOD removed. By comparison, when phosphorous isprecipitated in the manner of this invention, the excess total solidsremoved from the system (both the first and second aeration zones) maycomprise 0.8-l.6 lbs/lb. BOD removed.

It has been previously indicated that the partially treated effluentwater entering the second aeration zone should contain at least 25 ppm.BOD (residual carbonfood not removed in the first aeration zone),possible residual phosphorous pollutant, and unconsumedphosphorous-precipitating cation. When nitrification is an objective,this partially treated effluent water preferably contains less than 100ppm. BOD so as to permit growth of the nitrogen-consuming microorganismsin the second aeration zone as taught in my parent application Ser. No.249,159, incorporated herein to the extent pertinent.

When nitrification is not an objective, the second aeration zone may beused to a greater extent for BOD removal. In this mode of operation, arelatively high BOD content in the partially treated effluent water,such as 125 ppm., is achieved by maintaining the first aeration zonefood/biomass ratio near the upper limit of 2.5 pounds BOD /day poundMLVSS. The resultant high rate of unrecycled second solids from thesecond aeration zone is advantageous in suppressing the chemical solidsfraction of the total solids in the second aeration zone. This isdesirable because high chemical solids content tends to result in a highsuspended solids concentration in the product effluent water leaving thesecond aeration zone (discussed later in detail).

In addition to soluble residual carbonaceous and possible phosphorouspollutants, the partially treated effluent water entering the secondaeration zone also contains considerable suspended solids. In fact, thesuspended solids content is significantly greater than normally occursin oxygen gas staged activated sludge system wherein the food/biomassratio is comparable, i.e. 0.8-2.5, when phosphorous precipitatingcompounds are not introduced. This increased turbidity is notpredictable from the prior art as many investigators have observed thatphosphorous-precipitating compounds and flocculation and that heavierdosages reduce turbidity. The higher suspended solids concentration inthe partially treated effluent water is believed due to the heavyaccumulation of chemical solids in the total solids separated from thefirst aeration zone, which accumulation occurs after continuous,sustained operation, In effect, the system becomes overburdened withsmall positively charged particles which inhibit effective flocculationand permit both chemical and organic solids to persist in suspension inthe first aeration zone effluent. Unprecipitated cation of the addedcompound is also carried over into the second aeration zone. However,most of the phosphorous pollutant reacts chemically in the firstaeration zone in the presence of excess cation, and its resultantinsoluble salts are largely removed in the first stepclarification-sedementation.

Despite the large suspended solids increase in the partially treatedwater which occurs when sustained addition of thephosphorous-precipitating compound is practiced, it has beenunexpectedly discovered that a superlative effluent is produced in thesecond aeration zone when the operating conditions of this method aremaintained in the first and second aeration zones. As will beillustrated by the ensuing examples, the product effluent water containsonly about 10 ppm. suspended solids and very low residual levels of allpollutants: phosphorous (1 ppm.), nitrogen (1 ppm.) and BOD l4 ppm.).

The chemical solids accumulation in the second aeration zone depends atleast in part on the food/biomass ratio and thephosphorous-precipitating cation/phosphorous pollutant molar ratiomaintained in the first aeration zone. If the last mentioned ratio isrelatively high, e.g. 1.6, then relatively more cation carry-over to thesecond aeration zone will occur and the chemical solids will tend toincrease relative to other solids (biological). If thecation/phosphorous pollutant ratio in the first aeration zone isrelatively low, e.g. 1.2, the chemical solids concentration in thesecond aeration zone will also tend to be low, assuming that a polishingdose of the phosphorous-precipitating compound is not introduceddirectly to the second aeration zone. If the first zone food/biomassratio is relatively high, e.g. 2.4, then more BOD will be carried overto the second aeration zone and the biological solids (carbonaceous andnitrogenous) will tend to increase relative to chemical solids. Lowfood/biomass ratio in the first aeration zone will produce the oppositeeffect in the second aeration zone.

If necessary, additional phosphorous-precipitating compound may bedirectly introduced to the second aeration zone, so as to maintain thephosphorousprecipitating cation concentration so that the chemicalsolids/total solids weight ratio at least at 0.05 and less than 0.25.The function of the phosphorousprecipitating compound, whether carriedover from the first aeration zone or directly added to the secondaeration zone, is to aid flocculation the formation of a moderatefraction of positive changes in the liquor which, in the presence ofnormally occurring negatively charged biological solids, results ineffective capture" of substantially all the fine solids within large,settleable floc particles. The chemical solids accumulation in thesecond aeration zone is an indicator of the phosphorens-precipitatingcation concentration.

The food/biomass ratio in the second aeration zone is low on an absolutebasis (down to 0.15 pounds BOD /day X pound MLVSS) and relative to thefirst aeration zone (no more than one half). Low F/M is needed to obtainthe phosphorous precipitating cation/biomass relationship needed forexceptional clarity in the product effluent water. Also, ifnitrification is an object a low food/biomass ratio is necessary tosuppress production of carbonaceous solids and maintain a nitrifyingbiomass. lf nitrification is to be optimized, the food/biomass ratio inthe second aeration zone should be within the lower portion of theacceptable range, i.e. O.l5-0.5.

Only the minimum energy required for mixing and fluid recirculationshould be expended in the second aeration zone. Floc damage in this zonedue to excessive energy would be more detrimental to overall processperformance since the product effluent water is normally discharged fromthe second aeration zone clarifier without further opportunity forreflocculation. More particularly, the total energy expended in theliquor introductory flow section having liquor contact time of at leastminutes does not exceed 0.3 horsepower/1,000 U.S. gallons of secondaeration zone liquid capacity including a high shear part not exceeding0.25 horsepower/1,000 U.S. gallons (the upper limit of the firstaeration zone liquor terminal flow section).

The energy expenditure in the liquor terminal flow section must bemaintained as low as possible (for the above stated reasons) to achievethe necessary liquidsolids mixing and fluid recirculation. Inparticular, the total energy expended in a liquor terminal flow sectionof the second aeration zone having liquor contact time of at least 10minutes does not exceed 0.25 horsepower/1,000 U.S. gallons of secondaeration zone liquid capacity including a high shear part not exceeding0.20 horsepower/1,000 U.S. gallons. These low power levels are feasibleinasmuch as oxygen demand is very low in the terminal liquid flowsection so that the power requirement is established almost entirely byliquid-solids mixing. Also, the second aeration zone total solidsconcentration is lower than in thefirst zone, and the total solidscontains a lower fraction of heavy chemical solids thereby facilitatinglower power expenditure for liquid-solids mixing.

The total energy requirements of the liquor introductory flow section ofthe second aeration zone are usually greater than the liquor terminalflow section so that the mixing and fluid recirculation means in thelatter may often be operated at the lowest level of the total system,with the upstream means operated at progressively higher power levels.

The pH of the liquor in the second aeration zone is also maintained inthe range of 5.57.0 and preferably 5.5-6.5 by the dissolved CO in theliquor of a closed oxygen aerated systems and if necessary may be partially controlled by regulating the discharge of CO containingoxygen-depleted aeration gas from thisaeration zone. As in the firstaeration zone, the use of at least 50 percent oxygen (by volume) feedgas with a closed overhead gas space will normally permit themaintenance of pH in the desired range without the addition of pHlowering chemicals.

The total liquor contact time in the second aeration zone should notexceed 240 minutes, and may be less if nitrification is not anobjective. This time plus the first aeration zone liquor contact timeupper limit of 180 minutes affords a total system time not exceeding 420minutes, considerably less than comparable prior art activated sludgeaeration systems removing the same amount of BOD nitrogenous andphosphorous pollutants.

The dissolved oxygen concentration in the liquor terminal flow sectionis maintained at least at 2 ppm. to insure a sufficient oxygen masstransfer driving force and product effluent water which will increaserather than deplete the receiving water's oxygen supply.

As previously indicated, most but not all of the wastewaters phosphorouspollutant is removed as chemical solids in the firstaeration zone, andthe settled solids from the zone have a chemical solids/total solidsweight ratio of at least 0.25. The concentration of chemical solids inthe second aeration zone is lower, i.e. the chemical solids/total solidsweight ratio is less than 0.25.

Referring now more specifically to the drawings and FIG. 1, wastewatercontaining carbon food and soluble phosphorous pollutant, as for examplemunicipal sewage, enters the liquor introductory flow section of chamber10 comprising the first aeration zone, through conduit 11. A source (notshown) of oxygen comprising at least 50 percent oxygen is provided andthe oxygen gas is flowed therefrom through conduit 12 having controlvalve 13 therein to chamber 10. The latter is provided with gas-tightcover 14 to maintain an oxygen-enriched aeration gas environment overthe liquor. Recycling first solids are also introduced to chamber 10through conduit 15, although'the wastewater and first solids may bemixed prior to introduction in the chamber if desired.

Chamber 10 is designed so that its length is very large relative to itswidth and depth. For a given enclosure volume such geometry increasesthe velocity of liquor flow from the introductory flow section to theliquor terminal flow section, and suppresses backmixing of liquor fromdownstream sections into upstream sections. In order to achieve asignificant plug flow effect without liquor partitions, the length ofthe tank should be greater than its width and liquor depth dimensions.Usually the width and depth dimensions are similar and do not differ bymore than a factor of 2 or 2%. The length of the tank should preferablybe at least 10 times the larger of the width and depth dimensions. Thus,a suitable plug flow tank geometry, expressed by the dimensional ratio,length width liquid depth and illustrating the minimum preferred length,is 20 2 l.

The aforementioned streams are initimately mixed in chamber 10 by amultiplicity of submerged agitators 16a-c spaced longitudinally fromend-to-end of chamber 10 and driven by motors -1: through joining shaftmeans. Oxygen depleted aeration gas is withdrawn at longitudinallyspaced locations through conduits l9a-c by separate blowers 20a-c forcompression and return through conduits 21a-c to submerged spargers ordiffusers 22a-c preferably positioned below corresponding agitators16a-c. In this manner the aeration gas is continuously recirculated inintimate contact with the liquor in several longitudinally spacedsections in chamber 10. Blowers 20a-c are driven by motors (notillustrated), representing the expended fluid recirculation energy, andare preferably provided with controls to permit adjustment of therotation speed.

When there are a series of mixing-aerating devices spaced along anelongated tank without liquid partitions as for example chambers and 110of FIG. 1, the liquor terminal flow section of the zone (either first orsecond) will be the region of influence of the last device(s) in theseries, assuming that such region of influence provides at least theminimum 10-minute liquor contact time. The zone of influence of the lastdevice(s) is dependent upon its power relative to the power of theupstream adjacent mixing-aerating device(s). For example, with referenceto FIG. 1 assume the agitator 16c and sparger 220 in the terminalsection -of the first zone are located a distance A from the terminalwall of tank 10 and a distance B from agitation 16b and sparger 22b.Assume that the input power values to 16 plus 22c and to 16b plus 22bare X and Y, respectively. In the practice of this embodiment of theinvention, the section of influence of 160 and 220 will extend upstreamtoward 16b and 22b, a dimension C which is equal to B times X/ (X+Y),and the overall dimension of the terminal section in the longitudinalliquor flow direction is A C, or A B times X/(X+Y). By multiplying theoverall longitudinal dimension by the width and liquor depth of thetank, one obtains volume of the terminal liquor flow section, and bydividing this volume by the rate of liquor throughput (wastewater feedplus solids recycle), the liquor contact time in the terminal section isdetermined. By dividing the terminal section input power value X by thevolume of the terminal section, the power density is found (e.g.,HP/l,000 gal.).

A phosphorous-precipitating compound, either or both ferric chloride andalum, is introduced preferably in the form of aqueous solution to theliquor terminal flow section of chamber 10 through conduit 50 andcontrol valve 51. Chemical solids are thereby formed in addition to thebiological solids, and partially oxygenated liquor is discharged fromthe terminal flow section over weir 25 into overflow trough 26 andthence through discharge conduit 27. The oxygen depleted aeration gasmay be continuously or intermittently discharged from the overhead spaceof the liquor terminal flow section through conduit 23 having controlvalve 24 therein.

The partially oxygenated liquor in conduit 27 is introduced within acentral concentric baffle 28 of first clarifier 29. Baffle 28 preferablyextends from above the liquid level to a point intermediate to thislevel and the clarifiers conical bottom. Motor 30 drives a slowlyrotating rake 31 across the clarifier bottom to prevent coning of thedense settled solids. The supernatent liquid or partially treatedeffluent water, still with at least 25 ppm. BOD and unconsumedphosphorousprecipitating cation, overflows weir 32 into trough 33 and isdischarged through conduit 34. The first solids (comprising chemical andbiological solids) is withdrawn from the clarifier bottom throughconduit 35 1 and at least a portion thereof is pressurized by pump 36for recycling in conduit 15 to chamber 10 for inoculation of theincoming wastewater. Any first solids not needed for recirculation aredischarged through bottom conduit 37 having control valve 38 therein.

The partially treated effluent water from clarifier 29 comprises thesole liquid feed stream to the second zone. The apparatus previouslydescribed in the context of the first aeration zone may be substantiallyduplicated as the second aeration zone. In FIG. 1, elementscorresponding to those previously described have been identified by thesame number plus 100, and the second aeration zone operates in ananalogous manner to the first aeration zone, except for certainparameters discussed hereinafter in detail. In brief, the partiallytreated effluent water in conduit 34 having valve 350 therein, entersthe liquor introductory section of chamber comprising the secondaeration zone and is mixed therein with at least 50 percent (by volume)oxygen feed gas introduced through conduit 112, and second solidsrecycle introduced through conduit 115. Additional phosphorousprecipitating compound may be introduced through conduit 150 and controlvalve 151 therein, if needed to maintain the chemical solids/totalsolids weight ratio at least at 0.05 in-the second aeration zone.

Chamber 110 is designed in a manner analogous to chamber 10 to approachplug flow of liquor from endto-end That is, the length is very largecompared to its width and depth. As an illustration for approaching plugflow of liquor in a rectangular chamber, the length width liquor depthratios may be about 20 2 1. Also, longitudinally spaced submergedagitators 116a-c, and aeration gas recirculation assemblies 119, 120,121 and l22ac function in an analogous manner to their chamber 10counterparts. Oxygen-depleted overhead gas is released from the overheadspace in the liquor terminal flow section of chamber 110 through conduit123 and control valve 124 therein. Further oxygenated liquor isdischarged from the same liquor terminal flow section through conduit127 to second clarifier 129 operating in a manner very similar to firstclarifier 29. Product effluent water is discharged from the systemthrough conduit 140 and the second solids withdrawn from the bottomthrough conduit 135. At least part of the latter is recycled throughconduit by pump 136 to the liquor introductory section of chamber 110along with the partially treated wastewater.

The balance of the second solids are discharged through conduit 137 andcontrol valve 138 therein.

In a preferred embodiment of this invention, the first aeration zone andthe second aeration zone each comprise a multiplicity of separatesub-zones, wherein the oxygen gas, wastewater feed, and first solidsrecycle are all introduced to a first sub-zone as the liquorintroductory flow section of the first aeration zone for mixing andsimultaneous fluid recirculation therein to form a first partiallyoxygenated liquor and a first oxygendepleted aeration gas. They areseparately withdrawn and each introduced to a second sub-zone to form asecond partially oxygenated liquor and'second further oxygen-depletedaeration gas. These are in turn separately withdrawn from the secondsub-zone and each is introduced to any remaining sub-zones of the firstaeration zone for further mixing and fluid recirculation in the samecocurrent flow direction as the first and second sub-zones.

The phosphorous-precipitating compound is introduced to the finalsub-zone as the aforementioned liquid terminal flow section, and theaeration gas from the final sub-zone is released as theoxygen-depletedaeration gas.

Oxygen feed gas, partially treated effluent water and second solidsrecycle are all introduced to a first subzone as the liquor introductoryflow section of the second aeration zone for mixing and simultaneousfluid recirculation therein to form a first further oxygenated liquorand a first oxygen-depleted aeration gas. They are separately withdrawnand each introduced to a second sub-zone for further mixing andsimultaneous fluid recirculation to form a second further oxygenatedliquor and a second oxygen-depleted aeration gas. These are in turnseparately withdrawn from the second subzone and each introduced to anyremaining sub-zone of the second aeration zone for further mixing andfluid recirculation in the same cocurrent flow direction as the firstand second sub-zones, and aeration gas from the final sub-zone isreleased as the oxygen-depleted aeration gas.

Referring now more specifically to the FIG. 2 embodiment employing threesub-zones in the first aeration zone and two sub-zones in the secondaeration zone, elements corresponding to FIG. 1 elements have beenidentified by the same number. First aeration zone 10 is divided intothree separate compartments or sub-zones 10a, 10 b and 10c byintermediate partitions 42 and 45 extending from top to bottom.Restricted opening 43 in partition 42 below the liquor level providesflow of first partially oxygenated liquor from first sub-zone 10a tosecond sub-zone 10b, and restricted opening 44 in the aeration gas spaceprovides flow of first oxygen-depleted aeration gas from 10a to 10b incocurrent flow relation to the liquor. Similarly, restricted opening 46in partition 45 below the liquor level provides flow of second partiallyoxygenated liquor from second sub-zone 10b to third sub-zone 10c andrestricted opening 47 in the aeration gas space provides fiow of secondoxygen-depleted aeration gas from 1012 to 10c in cocurrent flow relationto the liquor. Third sub-zone 100 is the liquor terminal flow section offirst zone 10, assuming that the liquor contact time in 10c is at least10 minutes. If the 100 liquor contact time is less than 10 minutes, theliquor terminal flow section also includes 10b so that the 10b and 10ccontact times total at least l minutes.

Surface-type impellers 22a, 22b and 22c are provided in sub-zones a, 10band 100 respectively to throw sheets of liquor into the gas space forrecirculation against the gas and simultaneously perform the liquidsolidmixing function. That is, in the FIG. 1 embodiment, aeration gas isrecirculated against the liquor by pumps and reintroduced throughsub-surface spargers, while liquid-solid mixing is accomplished bysubsurface propellers. In the FIG. 2 embodiment, both fluid (liquor)recirculation and liquid-solid mixing are provided by the samemechanical device-motor driven surface impellers.

Third partially oxygenated liquor is withdrawn from the liquor terminalflow section 100 through conduit 27 joining first clarifier 29, and thephosphorousprecipitating compound is introduced thereto. As previouslyindicated, the precipitating reaction with the soluble ferric oraluminum cation is so rapid that very little contact time is neededupstream first clarifier 29.

Also, no mechanical mixing is needed in conduit 27 (often in the form ofan open trough), so no external energy is expended in the liquor flowsection of the phosphorous-precipitating compound introduction for thisembodiment.

The third oxygen-depleted gas from the liquor terminal flow sub-zone isdischarged through conduit 23 and control valve 24, and introduced tofirst sub-zone a of the second aeration zone as part of the oxygena'eration gas. This gas still contains a relatively high oxygenconcentration. e.g. 60-80% 0, by volume, assuming that the feed gas tothe first aeration zone is 90-100% 0 Any balance of the second zoneoxygen requirement is introduced to first sub-zone 1100 through conduit112 and control valve 113.

Second aeration zone 110 operates in a manner analogous to firstaeration zone 10 except that only one intermediate partition 142 isemployed to form first subzone 110a and second sub-zone l10b. That is,first fur ther oxygenated liquor flows from 110a to 1l0b throughsub-surface opening 143 in partition 142, and first oxygen-depleted gasflows from 110a to 11% through opening 144 in the overhead gas space.

Both clarifiers 29 and 129 operate in the same manner as in the FIG. 1embodiment. FIG. 2 does however illustrate the alternative of recyclingpart of the second solids to the first zone liquor introductory flowsection along with the first solids. As previously discussed, the secondsolids in conduit contains a substantially higher fraction of biological(organic) solids than the first solids in conduit 35. The highaccumulation of heavy chemical solids and the resulting high solidswaste rate from the first aeration zone tends to decrease theconcentration of microorganisms (hence the biological activity) in thefirst zone liquor. The latters microorganism concentration (MLVSS) maybe supplemented by diverting part of the second solids through conduit153 from return by pump 154 to conduit l5 and introduction to zone 10aalong with the first solids recycle.

The advantages of this invention were demonstrated in a series of pilotplant tests involving single zone 99 percent oxygen (by volume) feed gasaeration without addition of phosphorous-precipitating compound (TestNos. l and 4) and with alum addition (Test Nos. 2 and 3), two-zone 99percent 0 feed gas aeration without addition ofphosphorous-precipitating compound (Test Nos. 5 and 6), with addition ofan unsatisfactory phosphorous-precipitating compound, sodium aluminate(Test No. 7) and with the addition of alum as an embodiment of theinvention (Test No. 8). In each instance the aeration zone or zones hadat least three subzones arranged for cocurrent gas-liquor flow in themanner of FIG. 2, with a clarifier joining the liquor terminal portion.Each sub-zone was equipped with an oxygen gas sparger-impeller gas andliquor mixing unit driven by an electric motor. The sparger consisted ofrotating arms equipped with small diameter orifices through which theoxygen gas was recirculated, similar to FIG. 1 except that impeller 16and sparger 22 were mounted on a common shaft for rotation. The alum wasadded to only the last sub-zone of the single aeration zone in Test Nos.2 and 3, and the sodium aluminate and alum were added to only the lastsub-zone of first aeration zone in Test Nos. 7 and 8, respectively.

The pilot plants used in Test Nos. l-8 were four different types asfollows in Table A.

Test No.

Sub-zone vol., U.S. gallons 400 2050 50 45 No. of sub-zones 4 4 4 2 (1stzone) 3 (2nd zone) Total zone vol., U.S. gallons 1600 8200 200 90 (1stzone) 135 (2nd zone) Liquor depth, ft. 5.17 11.58 2.25 2 Sub-zonedimensions (ft. length 3.16 x 3.41 x 6.5 4.5 x 5.3 x 13.5 1.87 diam. x2.0 side wall 1.87 diam. x 2.9 side wall depth x width x height) 7 depthRotating sparger orifice size and 16. 13% 96, 14 A1, 6 96, 6

diameter, inches Horsepower, motors for rotating spargers (installed) 94(each sub-zone) 3 (1st sub-zone) 2 (other sub-zones) 230 V3 (eachsub-zone) 16 (each sub-zone) RPM, rotating spargers 190 200 (1stsub-zone) 250 (lat sub-zone of zones lan 2) M... -3... .5Q 9!EE.. 9..3.S9JJEIPZ9!), Horsepower, motors for re- :5 (1st sub-zone) 36 (1stsub-zone) ls (each sub zone) In (each sub-zone) circulating gas(installed) 16 (other subz0nes) 6 (other sub-zones) Impeller diameter,inches 14 14 6 6 During pilot plant operation, the oxygen feed gas wasintroduced to the overhead space of the first subzone of each zone andmaintained at slightly above atmospheric pressure and passed tosucceeding sub-zones through interconnecting piping. Gas purities weremeasured with an oxygen analyzer and the waste gas from each of the twozones during these tests was greater than percent (by volume). However,efficient oxygen utilization was not an object of these pilot planttests and more oxygen was wasted than would be tolerated in full-sizeplants. Moreover the pilot plants were not designed and operated tominimize energy consumption, and the observed horsepower values (per1,000 gallons liquid capacity) were substantially higher than would beexpected in full-size plants. The temperature of the mixed liquor was8-26C. and the pH in the range of 6.3-7.1. In addition to the monitoringof gas and liquor flows by appropriate metering and recording equipment,several important parameters were measured to determine the systemperformance. Daily composite samples were obtained for the feed water,

the first step clarifier effluent and the second step clarifiereffluent. Grab sample composite of the mixed li- 20 quor sludge weretaken daily for each step, and all analytical procedures for the sampleswere in accordance with the previously referenced Standard Methods forthe Examination of Water and wastewater. The data from the tests issummarized in Table B.

Table 1 summarizes the data from these tests. Test No. 1 shows that insingle zone high purity oxygen aerated plants designed in accordancewith U. S. Pat. No. 3,547,815, a high degree of carbonaceous removal(97% BOD may be achieved with low total suspended solids (MLSS) in theeffluent water (15 ppm). Test cal solids flocculation in a single zonecombined solids system. Test No. 4 involves the same wastewater andpilot plant as Test No. 3 but without alum addition. That is, alum wasadded to the mixed liquor during the TABLE B Test No. I 2 3 4 Zone 1Zone 2 Zone 1 Zone 2 Zone 1 Zone 2 Zone 1 Zone 2 Fee:

BODspp 227 114 99 113 110 69.5 119 27.7 110.2 27.4 106 45.3 TSS. ppm 76126- 112 123 126 54 150 49 124.3 65.8 118.5 70.3 TP. ppm 5.8 27 3.7 7.03.6 6.3 4.0 T1(N.ppm 38 28 11 22 27 22.5 27 19.5 20.2 15.2 19.7 18.0Effluent:

BOD-H ppm 8.0 13 36 32 69.5 22.5 27.7 11.8 27.4 28.0 45.3 14.1 TSS. ppm15 33 85 55 54 49 30 65.8 42.4 70.3 10.7 TP. ppm 3.8 7.3 1.9 3.6 2.8 4.01.0 TKN. ppm 1 25 15 i 8 19 22.5 5 19.5 3.3 15.2 1.6 18.0 1.3 Processconditions:

F/M. lbs. BODs/lb.

MLVSS 0.50 0.26 0 35 0.45 090 0.22 1.19 0 14 1 34 0.12 1 30 0.23 RTfirst subzone. min 23 22 26 26 ll 21 12 18 19 22 19 RT last sub'zone,min 23 22 26 26 11 21 12 18 12 19 22 19 RT total, min 92 88 105 103 6223 24 57 24 57 MLSS, ppm 7010 8160 4650 3700 6779 5961 6431 4134 58925425 5489 4805 MLVSS, ppm 5690 5410 3080 2780 5890 4365 4897 3128 39323612 3983 3128 DO. ppm 7.8 408.0 8.1 7.7 4.3 8.0 9.0 7.4 9.1 9.8 9.410.3 SVL ppm 42.1 53 55 24 9 24 28 32 4 27.2 51.8 23.4 47.6 Temperature(ave.),C 26 25 8 9 16 16 19 19 15.5 15.5 15 15 SRT, days 7.2 5 3.7 4.01.1 9.0 1.0 8 1 0.84 10 0.66 4.8 Test Duration, days 40 25 9 9 4 4 7 710 10 8 8 ISV. hr 10.6 7.5 5.5 8.6 7 4 11.0 7.9 9 0 8.2 8.9 5 8 Al/P,mole/mole 0 1.33 1.4 0 0 0 0 0 2.34 1.75 Total energy in last subzone.

HP/1,000 U.S. gallons 0.76 0.026 0.028 0.125 0.072 0.085 0 0.085 0.0650.092 0.123 High shear part of total energy in last sulrzone, HP/LOOOU.S. gallons 0.18 0.004 0.004 0.012 0.012 0.012 0.012 0.012 0.012 0.0120.012 Total energyin first sub-zone,

HP/1,000 U.S. gallons 0.81 0.054 0.872 0.164 0.118 0.144 0.184 0.1180.184 0.138 High shear part of total energy in first sulrzone, HP/1,000U.S. gallons 0.17 0.011 0.011 0.023 0.023 0.023 0.023 0.023 0.023 0.023Chemical solids/total solids in settled solids in treatment zone 0 0.440.23 0 0 0 0 0 0 31 0.28 0 27 0 23 Total phosphorous content. Totalnitrogen contact Total liquor contact time. feed plus recycle.

Sludge volume index. Sludge retention time. Settling rate of liquor toclarifier.

nine consecutive day Test No. 3 at an aluminum cation/phosphorouspollutant molar ratio of 1.4. Test No.

4 followed immediately for nine consecutive days and without alumaddition. It is evident that the effluent water total suspended solidsdropped significantly when alum addition was termined (85 to 55 ppm.).

Test Nos. 5-8 were all conducted in the same pilot plant having twoseparate aeration zones, and processing the same municipal wastewater.The only process parameter intentionally varied was the food/biomassratio (F/M). In every instance the ratio was relatively high in thefirst zone (0.90-1.34) and relatively low in the second zone (0.12-0.23)so that effective nitrification was achieved in Test Nos. 5 and 6 wherechemical solids were not produced. However, the first zone effluentwater in Test Nos. 5 and 6 contained relatively high total solidsconcentration (54 and 49 ppm.), and only marginal improvement wasobtained in the second aeration zone without alum addition. In Test No.5 the second zone effluent still contained 45 ppm. total suspendedsolids (a 16.6 percent reduction within that zone), and in Test No. 6the second zone effluent still contained 30 ppm. total suspended solids(a 33.3 percent reduction).

Test No. 7 shows that under the conditions of this process, sodiumaluminate NaAl(OH is not a satisfactory phosphorous-precipitatingcompound even though it has been commercially used in other wastewatertreatment systems for this purpose. In particular, heavy dosages wererequired in order to obtain complete reaction of the phosphorous (Al/Pmolar ratio 2.35), but the chemical solids in the first zone appear toflocculate and settle very poorly (effluent total suspended solids 65.8ppm.), and the improvement in the second zone effluent was limited (42.4ppm. or 35 percent).

In Test No. 8 conducted according to this invention, alum was added tothe liquor terminal flow section of the first zone so as to maintain analuminum cation/- phosphorous pollutant molar ratio (Al/P) of 1.75. Theabrupt increase in first zone effluent total suspended solidsconcentration from 54 ppm. (Test No. 5) to 70.3 ppm. is striking anddemonstrates the impairment in flocculation due to heavy accumulation ofchemical solids in that zone. Moreover the 45.3 ppm. BOD carryover fromthe first zone to the second zone reflected the high food/biomass ratio1.30 lbs. BOD /lb. MLVSS) in the first zone. Despite impairment offlocculation in the first zone, flocculation improved remarkably in thesecond zone. Suspended solids dropped 85 percent to only 10.7 ppm.,representing a 3 to 4 fold improvement over performance in the twozoneTest Nos. 5-7 and a 50-80 percent improvement over the single zone'TestNos. 1-4 in terms of the total suspended solids concentration in theeffluent water. To the best of my knowledge, such high overall effluentwater quality has not been achieved in wastewater treatment systemsemploying combined chemicalorganic sludges. Other tests have establishedthat results similar to those of Test No. 8 can be achieved when ferricchloride is substituted for alum as the phosphorous-precipitatingcompound.

It is also evident from comparison between Test No. 8 and singleaeration zone Test Nos. l-4 that the total aeration chamber volumerequired for two zone aeration to remove all pollutants according tothis invention is about the same as required to only remove carbonaceouspollutants in a single aeration zone. That is, the volume of aerationtankage is directly reflected in the liquor contact time (R T), otherparameters being equal, the total liquor contact time for Test No. 8 (2457 81 minutes) was lower than the Test Nos. l-4 (88 103 minutes).

Although certain embodiments of this invention have been described indetail, it will be appreciated that other embodiments are contemplatedalong with modifications of the disclosed features, as being within thescope of the invention.

I claim:

1. In a method for treating wastewater by aeration in contact withactivated sludge, settling sludge from the aeration and recycling sludgeto the aeration zone as said activated sludge wherein the carbon food insaid wastewater is biochemicallyv oxidized with at least 50 percentoxygen (by volume) feed gas, the improvement of phosphorous pollutantremoval comprising: introducing a wastewater feed stream containingcarbon food and soluble phosphorous pollutant, at least 50 percentoxygen (by volume) feed gas and first solids recycle to a first aerationzone having a closed overhead gas space and mixing the fluids in saidfirst aeration zone while simultaneously recirculating one fluid againstthe other fluids; introducing a phosphorousprecipitating compoundselected from the group consisting of ferric chloride and-aluminumsulfate; said introducing, mixing and recirculating being at rates suchthat: (a) insoluble chemical solids are produced including precipitatingphosphorous salt, and the phosphorous-precipitating cation/phosphorouspollutant molar ratio is 1.2-1.8, (b) the food/biomass ratio ismaintained at 0.8-2.5 pounds BOD lday X pound volatile suspended solids(MLVSS), (c) the volatile suspended solids concentration (MLVSS) is atleast 2,000 ppm., ((1) the total mixing and fluid recirculation energyexpended in a liquor terminal flow section of said first aeration zonehaving liquor contact time of at least 10 minutes does not exceed 0.3horsepower/1,000 U.S. gallons of terminal flow section liquid capacityincluding a high shear part of said total mixing and'recirculationenergy not exceeding 0.25 horsepower/1,000 U.S. gallons, (e) the totalmixing and fluid recirculation energy expended in the liquor flowsection of said phosphorous-precipitating compound introduction does notexced 0.3 horsepower/1,000 U.S. gallons of said liquor flow sectionliquid capacity including a high shear part of said total mixing andrecirculation energy not exceeding 0.25 horsepower/1,000 gallons, (f)the dissolved oxygen concentration in said liquor terminal flow sectionis at least 2 ppm., (g) the pH of said liquor in saidfirst aeration zoneis 5.5-7.0, and (h) the total liquor contact time in said first aerationzone does not exceed minutes; releasing oxygen-depleted aeration gas ofat least 20 percent oxygen (by volume) content from the first aerationzone overhead gas space; discharging partially oxygenated liquor fromsaid first aeration zone and separating same into partially treatedeffluent water still containing at least 25 ppm. BOD; and unconsumedphosphorous-precipitating cation, and settled solids having a chemicalsolids/total solids weight ratio of at least 0.25; returning part of thesettled solids to said first aeration zone as said first solids recycle;introducing, said partially treated effluent water, at least 50 percentoxygen (by volume) feed gas, and second solids recycle to a secondaeration zone having a closed overhead gas space and mixing the fluidsin said second aeration zone and maintaining thephosphorous-precipitating cation concentration so that the chemicalsolids/total solids weight ratio in'said second aeration zone is atleast 0.05 while simultaneously recirculating one fluid against theother fluids at rates such that: (i) additional insoluble chemicalsolids are formed from said phosphorous-precipitating compound, (j) thefood/biomass ratio is maintained at 0. l -0.8 pounds BOD /day poundvolatile suspended solids (MLVSS) and the ratio of first to secondaeration zone food/biomass ratio is at least 2, (k) the total mixing andfluid recirculation energy in the liquor introductory flow section ofsaid second aeration zone having liquor contact time of at least minutesdoes not exceed 0.30 horsepower/1,000 U.S. gallons of introductory flowsection liquid capacity including a high shear part of said total mixingand fluid recirculation energy not exceeding 0.25 horsepower/1,000 U.S.gallons, (l) the total mixing and fluid recirculation energy expended ina liquor terminal flow section of said second aeration zone having aliquor contact time of at least 10 minutes does not exceed 0.25horsepower/1,000 U.S. gallons of terminal flow section liquid capacityincluding a high shear part of said total mixing and fluid recirculationenergy not exceeding 0.20 horsepower/1,000 U.S. gallons, (m) the pH ofsaid liquor in said second aeration zone is 5.5-7.0, (n) the dissolvedoxygen concentration in said liquor terminal flow section is at least 2ppm. and (o) the total liquor contact time in said second aeration zonedoes not exceed 240 minutes; releasing oxygen-depleted aeration gas ofat least 20 percent (by volume) content from the second aeration zoneoverhead gas space; discharging further oxygenated liquor from saidsecond aeration zone and separating same into product effluent water andsettled solids having chemical solids/total solids weight ratio of lessthan 0.25; and returning part of the settled solids to said secondaeration zone as said second solids recycle.

2. A method according to claim 1 wherein said wastewater feed streamalso contains nitrogen food, the food/biomass ratio (b) in said firstaeration zone is less than 1.5 pounds BOD /day X pound volatilesuspended solids (MLVSS), the partially treated effluent water from saidfirst aeration zone contains less than 100 ppm. BOD and at least most ofsaid nitrogen food, the food/biomass ratio (j) in said second aerationzone is less than 0.5 pounds BOD /day pound volatile suspended solids(MLVSS), the volatile suspended solids population of said furtheroxygenated liquor comprises both 2-40 percent nitrogen-consumingmicroorganisms and 98-60 percent carbon-consuming microorganisms plusnon-viable material so that substantial nitrification occurs in saidsecond aeration zone.

3. A method according to claim 1 wherein the total mixing and fluidrecirculation energy expended in the liquor introductory flow section ofsaid first aeration zone is at least 1.1 times the total mixing andfluid recirculation energy expended in said liquor terminal flow sectionof said first aeration zone.

4. A method according to claim 1 wherein the phosphorous-precipitatingcompound is introduced in said liquor terminal flow section of saidfirst aeration zone.

5. A method according to claim 1 wherein additionalphosphorous-precipitating compound is introduced to said second aerationzone.

6. A method according to claim ll wherein said first aeration zone andsaid second aeration zone each comprise a multiplicity of separatesub-zones; said oxygen feed gas wastewater feed stream and first solidsrecycle are all introduced to a first sub-zone as said liquorintroductory flow section of said first aeration zone for mixing andsimultaneous fluid recirculation therein to form a first partiallyoxygenated liquor and a first oxygen-depleted aeration gas, said firstpartially oxygenated liquor and said first oxygen-depleted aeration gasare separately with-drawn and each introduced to a second sub-zone forfurther mixing and simultaneous fluid recirculation to form a secondpartially oxygenated liquor and second further oxygen-depleted aerationgas, said second partially oxygenated liquor and said second furtheroxygen-depleted aeration gas are separately withdrawn from said secondsub-zone and each introduced to any remaining sub-zones of said firstaeration zone for further mixing and fluid recirculation in the samecocurrent flow direction as said first and second sub-zones, saidphosphorous-precipitating compound is introduced to the final sub-zoneas said liquor terminal flow section, the aeration gas from the finalsub-zone is released as said oxygen-depleted aeration gas; said oxygenfeed gas, partially treated effluent water and second solids recycle areall introduced to a first sub-zone as said liquor introductory flowsection of said second aeration zone for mixing and simultaneous fluidrecirculation therein to form a first further oxygenated liquor and afirst oxygen-depleted aeration gas, said first further oxygenated liquorand said first oxygen-depleted aeration gas are separately withdrawn andeach introduced to a second sub-zone for further mixing and simultaneousfluid recirculation to form a second further oxygenated liquor and asecond oxygendepleted aeration gas, said second further oxygenatedliquor and said second oxygen-depleted aeration gas are separatelywithdrawn from said second sub-zone and each introduced to any remainingsub-zones of said second aeration zone for further mixing and fluidrecirculation in the same cocurrent flow direction as said first andsecond sub-zones, the aeration gas from the final sub-zone is releasedas said oxygen-depleted aeration gas.

7. A method according to claim 1 wherein the total A mixing and fluidrecirculation energy expended in said first aeration zone liquorterminal flow section does not exceed 0.25 horsepower/1,000 U.S. gallonsincluding a high shear part of such energy not exceeding 0.20horsepower/1,000 U.S. gallons.

8. A method according to claim 1 wherein the total mixing and fluidrecirculation energy expended in said second aeration zone liquorterminal flow section does not exceed 0.20 horsepower/1,000 U.S. gallonsincluding a high shear part of such energy not exceeding 0.15horsepower/1,000 U.S. gallon.

9. A method according to claim l.wherein the total mixing and fluidrecirculation energy expended in the first aeration zone liquorintroductory flow section does not exceed 0.50 horsepower/1,000 U.S.gallons.

10. A method according to claim 1 wherein the food/biomass ratio (b) insaid first aeration zone is less than .15 pounds BOD /day X poundvolatile suspended solids (MLVSS).

11. A method according to claim 2 wherein the food/biomass ratio (g) insaid second aeration zone is less than 0.5 pounds BOD /day poundvolatile suspended solids (MLVSS).

12. A method according to claim 1 wherein ferric chloride is saidphosphorous-precipitating compound.

13. A method according to claim 1 wherein alumiof the liquor in saidfirst and second aeration zones is 6.0-6.7.

15. A method according to claim 1 wherein the settled solids from saidfirst aeration zone has a chemical num sulfate is saidphosphorous-precipitating com- 5 solids/total solids weight ratio ofless than 0.50.

UNITED STATES PATENT OFFIC CERTIFFCATE OF 3,764,524 October 9, 1973Patent No. Issue Date I Inventor(s) Micahel J. Staukewich, Jr.

it is certified that error appears in the above-identified potent andthat said Letters Patent are hereby corrected as shown below:

Claim 1, column 24, line 46 "exced" shotlld read ---ex ceed-- Claim 10,column 2 line 63 ".15" should read l.5--.

Claim 11, column 26, line 65, "2" should read Signed and sealed this 5thday of February 1974.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. RENE D. TEGTMEYER Attesting Officer ActingCommissioner of Patents

2. A method according to claim 1 wherein said wastewater feed streamalso contains nitrogen food, the food/biomass ratio (b) in said firstaeration zone is less than 1.5 pounds BOD5/day X pound volatilesuspended solids (MLVSS), the partially treated effluent water from saidfirst aeration zone contains less than 100 ppm. BOD5 and at least mostof said nitrogen food, the food/biomass ratio (j) in said secondaeration zone is less than 0.5 pounds BOD5/day X pound volatilesuspended solids (MLVSS), the volatile suspended solids population ofsaid further oxygenated liquor comprises both 2-40 percentnitrogen-consuming microorganisms and 98-60 percent carbon-consumingmicroorganisms plus non-viable material so that substantialnitrification occurs in said second aeration zone.
 3. A method accordingto claim 1 wherein the total mixing and fluid recirculation energyexpended in the liquor introductory flow section of said first aerationzone is at least 1.1 times the total mixing and fluid recirculationenergy expended in said liquor terminal flow section of said firstaeration zone.
 4. A method according to claim 1 wherein thephosphorous-precipitating compound is introduced in said liquor terminalflow section of said first aeration zone.
 5. A method according to claim1 wherein additional phosphorous-precipitating compound is introduced tosaid second aeration zone.
 6. A method according to claim 1 wherein saidfirst aeration zone and said second aeration zone each comprise amultiplicity of separate sub-zones; said oxygen feed gas wastewater feedstream and first solids recycle are all introduced to a first sub-zoneas said liquor introductory flow section of said first aeration zone formixing and simultaneous fluid recirculation therein to form a firstpartially oxygenated liquor and a first oxygen-depleted aeration gas,said first partially oxygenated liquor and said first oxygen-depletedaeration gas are separately withdrawn and each introduced to a secondsub-zone for further mixing and simultaneous fluid recirculation to forma second partially oxygenated liquor and second further oxygen-depletedaeration gas, said second partially oxygenated liquor and said secondfurther oxygen-depleted aeration gas are separately withdrawn from saidsecond sub-zone and each introduced to any remaining sub-zones of saidfirst aeration zone for further mixing and fluid recirculatioN in thesame cocurrent flow direction as said first and second sub-zones, saidphosphorous-precipitating compound is introduced to the final sub-zoneas said liquor terminal flow section, the aeration gas from the finalsub-zone is released as said oxygen-depleted aeration gas; said oxygenfeed gas, partially treated effluent water and second solids recycle areall introduced to a first sub-zone as said liquor introductory flowsection of said second aeration zone for mixing and simultaneous fluidrecirculation therein to form a first further oxygenated liquor and afirst oxygen-depleted aeration gas, said first further oxygenated liquorand said first oxygen-depleted aeration gas are separately withdrawn andeach introduced to a second sub-zone for further mixing and simultaneousfluid recirculation to form a second further oxygenated liquor and asecond oxygen-depleted aeration gas, said second further oxygenatedliquor and said second oxygen-depleted aeration gas are separatelywithdrawn from said second sub-zone and each introduced to any remainingsub-zones of said second aeration zone for further mixing and fluidrecirculation in the same cocurrent flow direction as said first andsecond sub-zones, the aeration gas from the final sub-zone is releasedas said oxygen-depleted aeration gas.
 7. A method according to claim 1wherein the total mixing and fluid recirculation energy expended in saidfirst aeration zone liquor terminal flow section does not exceed 0.25horsepower/1, 000 U.S. gallons including a high shear part of suchenergy not exceeding 0.20 horsepower/1,000 U.S. gallons.
 8. A methodaccording to claim 1 wherein the total mixing and fluid recirculationenergy expended in said second aeration zone liquor terminal flowsection does not exceed 0.20 horsepower/1, 000 U.S. gallons including ahigh shear part of such energy not exceeding 0.15 horsepower/1,000 U.S.gallon.
 9. A method according to claim 1 wherein the total mixing andfluid recirculation energy expended in the first aeration zone liquorintroductory flow section does not exceed 0.50 horsepower/1,000 U.S.gallons.
 10. A method according to claim 1 wherein the food/biomassratio (b) in said first aeration zone is less than 1.5 pounds BOD5/day Xpound volatile suspended solids (MLVSS).
 11. A method according to claim1 wherein the food/biomass ratio (g) in said second aeration zone isless than 0.5 pounds BOD5/day X pound volatile suspended solids (MLVSS).12. A method according to claim 1 wherein ferric chloride is saidphosphorous-precipitating compound.
 13. A method according to claim 1wherein aluminum sulfate is said phosphorous-precipitating compound. 14.A method according to claim 1 wherein the pH of the liquor in said firstand second aeration zones is 6.0-6.7.
 15. A method according to claim 1wherein the settled solids from said first aeration zone has a chemicalsolids/total solids weight ratio of less than 0.50.